Fifth-Generation (5G) air-interface protocols, also referred to as New Radio (NR), are currently being developed by the Third Generation Partnership Project (3GPP). The beam-based rank-agnostic CBSR approach used in Long Term Evolution (LTE) cannot be directly re-used for NR as the same beam-based quantity is not a constituent component of the precoders for all ranks. Namely, the precoder codebooks for ranks 3 and 4 in LTE are constructed with a different beam-based quantity than the remaining ranks. An alternative is to use the pre-LTE full dimension multiple-input, multiple-output (FD-MIMO) CBSR approach based on precoding matrix indicator (PMI)-based per-rank CBSR, however this would lead to around 8 times more signaling overhead compared to LTE.
NR codebooks use a generalized co-phasing behavior that is different from prior art codebooks in LTE, as there are now two parameters (φ_n and θ_p) used to co-phase the two-dimensional discrete Fourier transform (2D DFT) beams. The co-phasing behavior depends on the number of Channel State Information Reference Signal (CSI-RS) ports used in the codebook and may be different for codebooks of different ranks. Consequently, new mechanisms are needed in NR for codebook restriction of co-phasing and rank.